Study of lubricating coatings for glass containers with optical microscopy and FTIR microscopy

Optical microscopy, transmission and reflection infrared spectroscopy, contact angle and weight loss on annealing are used to establish the thickness and homogeneity of the organic coatings used industrially to lubricate glass containers. As shown by a range of experiments and mathematical simulations, infrared spectroscopy is suitable to detect in situ the C-H bands of the hydrocarbon material near 2900 cm^-1 and to quantify the organic coverage. Useful results are obtained using a FTIR spectrometer equipped either with a specular reflectance accessory or with an infrared microscope. Due to the different size of the area analysed (about 1 cm² versus 1×10^-4 cm²), the first setup is suitable to monitor the average coverage, the second to check the local distribution. Other absorption bands of the coating, particularly those associated with the carbon-oxygen bonds and potentially suitable to provide information on the organic-to-glass bonds, are too weak to be detected. The study provides for the first time an estimate of the thickness of the cold end coatings (generally within 100 nm) and highlights their inhomogeneous distribution and patchy structure.